Note: Descriptions are shown in the official language in which they were submitted.
~3~
METHOD OF AND APPARATUS FOR CO;`JTINUOUS
CASTIN~ OF METAL STRIP
Background oE the Invention
This invention relates to casting of metal sheet
or strip, and more particularly to an improved method
of and apparatus for the high speed direct casting of
thin metal sheet or strip in a continuous or semi-
continuous operation.
The metals industry has, for many years, sought
to develop a commercially acceptable process for the
direct casting of metals into thin sheet or strip
(hereinafter, strip) in a semi-continuous or continuous
operation, These efforts have been intensified in
recent years and substantial research and development
has been sponsored both by indus-try and by various
governments, For example, the United States Depart-
ment of Energy has awarded a 2.6 million dollar con-
. tract to Westinghouse Electric Corporation and Armco
Inc. jointly for development of a roller casting
process for forming strip only 3 inches in width, and
a 30 million dollar contract to United States Steel
Corpoxation and Bethlehem Steel Corporation jointly
for the design and construction of a pilot plant for
strip casting.
Efforts to develop a commercially acceptable
process for direct casting metal strip have continued
Il'~ ' ~
32~l~
-2-
-throughou-t most of this c~ntury. For example, Brltish
Patent No. 6,630 discloses the concep-t of flowing molten
metal at a constant rate onto a moving cooled surface
to be solidified, and drawiny the metal in the form of
a thin strip from the cooled surface in a continuous
process. Other patents disclosing and describing im-
provements and varia~ions of this basic concept include,
for example, U.S. Patent Nos. 4,479,528; 4,449,568 and
3,381,736.
A variation of the above concept involves con-
ducting molten metal from a tundish through a restricted
outlet so as to provide a convex meniscus at the outlet
opening, with the molten metal being drawn from the
opening by contacting the meniscus with a moving cooled
surface. Examples oE patents disclosing this concept
include British Patent No. 20,518 and U.S. Patent Nos.
3,522,836 and 3,605,863.
The concept oE flowing a stream oE molten metal
into the nip of a pair of spaced, counter-rotating chilled
rolls to produce an elongated strip rolled and chilled on
both surfaces~is disclosed, for example, in U.S. Patent
Nos. 4,212,344 and 4,337,087 and is described as known
prior art in Japanese Published Application 58-41656.
The use of a chilled roll surface partially sub-
merged in a molten metal bath and driven to withdraw
a strip or filament of metal solidified and adhered
to the chilled surface is disclosed in U.S. Patent Nos.
~ .
3,540,517 and 3,812,901. The use o~ a pair of counter-
rotating rolls having chilled surfaces partially sub-
merged in a molten metal bath to withdraw a continuous
strip through the nip of the rolls is shown in U.S.
Patent Nos. 3,823,762 and 3,857,434.
The use of travelling molds in the form of endless
chilled belts or of ch.illed mold sections connected
in caterpillar-track fashion for the continuous casting
of metal is also known and is commercially used in the
production of pla.te or thin slabs. Such devices may
use a single casting mold or belt as shown in U.S.
Patent Nos. 2,348,178; 3,381,739; 4,274,473; and 4,323,419,
or a pair of cooperating endless molds or belts, as
shown in U.S. Patent Nos. 3,642,055 and 4,061,177.
The combination of an endless belt or casting mold
with a roll contacting the opposed surface of the
cast strand lS also known as shown, for example, in
U.S. Patent Nos. 4,202,404 and 4,372,368, Swiss Patent
:No. 622,725, and French Patent No. 1,364,717.
~32~
_
The use of a gas jet directed onto the free
surface of a pool of molten metal to create a standing
wave contacting a cooled casting wheel surface is
dlsclosed in U.S~ Patent 3,863,700.
It is also known to spray a cooling fluld such
as an lnert gas or a coollng llquld onto the exposed
molten surface of a partlally solldlfled band of
metal movlng on a chilled casting surface to enable
the continuous casting of a thicker gauge plate and
to enable subsequent shaping of the exposed top` surface
by a shaping roll, as illustrated for example in the
publlshed Japanese appllcatlon and the Swiss patent
mentioned hereinabove.
U.S. Patent 4,282,921 discloses a process for
melt spinnlng narrow metalllc ribbons by dlrectlng a
jet of molten metal onto a movlng chill block
surface and dlrectlng a gas stream confluent with
and surrounding the molten me~al jet so that the gas
surrounds and bears upon the metal puddle near the
point )f lmpingement on the movlng chllled surface
to stablllze the puddle as the ribbon ls formed.
.
32: L~
--5--
The use of air knives for controlling the thick-
ness of a liquid metal coating on a solid substrate
is well known and widely used particularly in hot
dip galvanizing and aluminizing of metal strip. Such
air knives conventionally include an elongated hollow
manifold extending transversely of and in close proximity
to the emerging elongated substrate at a point spaced
above the coating metal bath. An elongated, narrow
nozzle opening extends along the manifold and faces
in the direction of the coated substrate. Gas, under
pressure t' discharged from the nozzle acts as a pressure
dam which, depending upon characteris-tics of the jet
: including the direction, velocity and mass of the gas
and the proximity of the nozzle outlet to the li~uid
coating material, limits the thickness of the li~uid
coating carried past the air knife.
In such hot dip metal coating operation, the metal
: : substrate is conventionally passed through a preheating
furnace and led:directly into the mol~en metal bath
so that tbe substrate is at a temperature which will
mainlain the coating in a liquid state for a substantial
:distance past the air knifeJ with solidifi~ation normally
:
taking place from the exposed coating surface inward.
.
i32~
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A detailed technical description and analysis of the
hot dip metal coating on metal strip is presented by
John A. Thornton and Hart F. Graff, An Analytical Des-
cription of the ~et Finishing Process for Hot Dip
Metallic Coatings on Strip, Metallurgical Transactions
B, pages 607 - 618, December 1976.
D~spite the continued eforts by the metals in-
dustry, applicant is unaware of any previous process
which is capable of producing commercially acceptable
cast metal strip in a continuous high speed process. It
is to be understood that the term "continuous" as used
herein is intended to include a semi-continuous direct
strip casting operation.
It is the primary object of the present invention
to provide an improved process for the direc-t continuous
casting of metal strip.
It is another object of the present invention to
provi.de an improved apparatus and process for contin-
uously producing thin cast metal strip uti.lizing a melt
drag technique.
Another object is to provide apparatus for and
method of continuously casting thin metallic strip having
a more uniform cross sectional shape and a good top
surface finish.
~gi3~
Another object is to provide such a method and
apparatus which is capable of high speed production of
cast metal strip in a continuous commercial operation.
Another object is -to provide a method and apparatus
for the high speed casting of metal strip having a thick-
ness which can be varied over a wide range.
Another object is to provide such a method and
apparatus for producing cast metal strip which is sub-
stantially free of surface defects and inclusions.
The foregoing and other objects and featurès of the
invention are achieved in accordance with the present
in~ention which enables metal strip to be cast from a
supply of molten metal by a continuous process wherein
the molten metal is brought into contact with a cooled
moving casting surface whereby a continuous strand of
the metal solidifies on and adheres to the cooled surface
to be withdrawn from the molten metal supply. An air
knife supported adjacent to the surface of the molten
~metal supply directs a thin low pressure air jet
into contact with the surfaces of the molten metal
supply and the emerging strand at the point of emergence
of the solidifying strand. The position and direction
of the fluid jet, the shape of the outlet nozzle and
the gas pressure are controlled to shape the free
~632~
surface of the strand as it exits from the molten metal
supply and to prevent o~ides,slag or other material on the
molten metal supply surface from adhering to liquid metal
on the surface of the strand while at the same time con-
trolling strip thickness and strip profile by limitingthe amount and distribution of liquid metal adhering to
the free top surface of the partially solidified strand.
In accordance with a preferred embodiment of the
invention, the casting surface is the cylindrical
outer surace of a casting wheel or drum which is
supported and driven for rotation about a fixed
horizontal axis. A tundish supported adjacent to the
casting wheel has an open end contoured to fit in close
conformity with and be effèctively closed by a portion
of the casting wheel surface. Molten metal is con-
tinuously supplied to the tundish to maintain a
substantially constant depth vf molten metal in contact
with the rotating chilled casting surface. As the cast-
ing su.rface moves upward through the molten metal the
metal wets and:adheres to the chilled surface and is
quickl~ solidified, with the solidified strand increas-
ing in thickness progressively until it emerges from
the top surface of the metal in the tundish.
~Z~;3~
g
A gas discharge nozzle assembly, hereinafter
referred -to an an air knife, is supported above the sur-
face of the metal in the tundish and has an elongated
nozzle outlet positioned and oriented to direct a
low speed jet of gas onto the molten metal surface
along the line vf intersection of the surface of the
molten metal and the casting surface, i.e., the point
at which the strand emerges from the molten metal. The
gas jet establishes a depression in the surface of the
molten metal in the tundish adjacent to the emerging
strand and produces a standing wave adjacent to the
depression. This standing wave and the gas from the
jet sweeping over its surface prevent oxides, slag and
other material on the molten metal surface from con-
tacting the exposed wet surface of the emerging strandadhering to and moving with the cooled casting surface.
The fluid jet has a component of motion generally
; perpendicular to the e~posed surface of the strand at
the point of emergence from the molten metal in the
20~ tundish.` At thls~point, the strand is substantially
solidified, but the exposed top surface has a layer
of liquid metal adhering thereto. The velocity,
mass and direction of the gas in the jet are con-
trolled so that the gas acts upon and shapes this
,
~26~
--10--
liquid layer to produce the desired cross-sectional
shape and surface finish. The jet is also effective
to limlt the thickness of the liquid layer and to this
ex-tent the thickness of -the strand. ~s previously
stated, however, the emerging strand is solidified
throughout at least a major portion of its thickness
and the final strip thickness will be determined by a
combination of factors.
The chilled casting wheel surface rapidly quenches
the metal contacting the;surface so that the strand is
completely solidified very quickly after passing beyond
the jet from the air knife. Thus, the top surface
finish and shape imparted to the strip by the controlled
jet is maintained as a result of the rapid solidification.
Strip thickness can also be varied independently of
the air knife by varying the time of exposure of the
- molten metal to the chilled casting wheel surface by
either varying the speed of the casting wheel or the
~ length of the wheel surface which at any given time is
20~ exposed to the molten metal in the tundish.
Other features and advantages of the invention wlll
be apparent from the detailed description contained
~hereinbelow, taken in conjunction with the drawings, in
which:
;32~
--11--
Fig. 1 is a schematic elevation view, partially
in section, of a strip casting apparatus according to
the present invention;
Fig. 2 is an enlarged fragmentary sectional view,
in elevation, oE a portion of the apparatus shown
in Fig. l;
Fig. 3 is a top plan view of the apparatus shown
in Fig. 2;
Fig. 4 ls a view taken along lines A_4 of Fig. 1
and showing the air knife assembly;
Fig. 5 is an enlarged sectional view taken along
lines 5_5 of ! Fig. 4;
Fig. 6 is a sectional view taken along lines
6-6 of Fig. l; and
Flg. 7 is a view similar to Fig. 2 and showing
: a prior art strip casting apparatus;
Referring now to the drawings in detail, a melt
drag strip casting apparatus embodying the present
lnVentiOIl lS illustrated schematically in ~ig. 1 and
designated generally by the reference numeral 10.
: '
:~
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-12-
In this embodiment of the invention, the apparatus in-
cludes a casting wheel or drum 12 having a cylindrical,
cooled outer surface 14 upon which the metallic
strip 16 is cast. A tundish assembly 18 is supported
S in close proximity to the casting wheel 12 in position
to contain a supply of molten metal 20 and to maintain
a uniform depth of the molten metal in contact with the
casting surface 14 of wheel 12.
In the preferred embodiment of the invention, the
casting wheel 12 is internally cooled with circulating
water or other cooling fluid to rapidly extract heat
through the peripheral surface 1~ to rapidly c~uench
. and solidify lic~uicl metal contacting the peripheral
casting surface as the surface rotates upward
through the molten metal supply 20 in tundish 18.
. Internally cooled casting drums are kno~n, for example
from U.S. Patent No. 2,348,178, and as schematically
illustrated in Fig. 6 may comprise a hollow drum made
up of a pair of end flanges 22, 24 and an outer
peripheral rim 26, the outer surface of which defines
~ the external casting surface 14. A central hub 28
.~ :
~3~
-13-
supported within the hollow drum has axially and
radially extending connecting inle-t passages 30, 32,
respectively, communicating with the annular space
34 between hub 28 and outer rim 26, and radially
and axially extending connecting passages 36, 38
communicating with the annular space 34 to provide
an outlet for cooling water.
The rim 26 of casting wheel 12 may be formed of
any suitable metal material having a sufficiently
high and uniform thermal conductivity and good wear
resistance. For example, casting wheels having a cast-
ing surEace of cQpper, steel, and aluminum and alloys
of these metals have been successfully employed for
the high speed casting of strip metal in the apparatus
of Figs. 1 - 6. The casting surface 14 may be sub-
stantially smooth but preferably is mechanically
roughened or grooved as suggested in U.S. Patent No.
4,250,950 or French Patent No. 1,364,717. Suitable
bearings 40 support the wheel 12 for rotation about a
20~ fixe~ horizontal axis on a rigid support frame structure
indicated in Pig. 1~ by the reference numeral 42.
Suitable varlable speed drive means, such as an
- ~ electric motor, not shown, acting through a reduction
gear mechanism 44 and a drive chain 46 drives wheel 12
:
;32~
-14-
at the desired speed about its fixed axis. Other suit-
able drive means may, of course, be employed instead
of the schematically illustrated chain drive.
The tundish 18 is supported in fixed relation to
casting wheel 12 by suitable frame structure illustrated
schematically at 48. The tundish is constructed from
a high strength, thermally insulating material such as
a cast ceramic material, or a rigid metal frame
structure lined with a suitable refractory material,
to minimiæe heat loss from the molten metal supply
20 contained within the tundish during operatlon.
In the illustrated embodiment, the tundish is made up
of a substantia1ly flat horizontally extending bottom
wall 50, a pair of spaced upward extending side walls
52, 53 and an end wall 54, with the end of the tundish
opposite end 54 being open to permit the molten metal
20 to flow into direc~ contact with the adjacent outer
~peripheral surface 14 of wheel 12. Thus, the wheel 12
acts as a wall or dam to prevent the flow of molten
20 : meta] out of the open end of the tundish. The end
of bottom wall 50,~and portions of the ends of side
~walls 52 - 53 are contoured to closely conform to the
:
contour of peripheral surface 14, with the spacing
.
.:, : ,
~632~4
-15-
between the tundish bottom walls and side walls and
the wheel surface 14 being so small as to prevent the
flow o~ liquid metal therebetween. In practice, it has
been Eound that a spacing of about 0.10 to l.00 mm.
between surface 14 and the adjacent tundish structure
is satisfactory.
A transverse partition wall 60 having its bottom
edge spaced above the top surface of bottom wall 50
is located in the tundish in spaced relation to end
: lO wall 54 to define a receiving chamber 62 for receiving
: ~ molten metal from a suitable source such as a ladle
or runner indicated generally at 64. A vertically
movable transverse wall or weir 66 also extends between
~ side walls 52, 53 at a location between the fixed wall
60 and the open end of the tundish, and cooperates with
side walls 52, 53 and partition wall 60 to define a
surge chamber 68. The bottom edge of wall 66 is spaced
above the top surface of bottom wall 50 during operation
~ of the apparatus for casting strip t~ permit a controlled
flo~ of liquid metal therebeneath to maintain a sub-
stantially uniform level~of metal in thè casting chamber
70. Suitable means, not shown, is provided for ver-
tically adjusting the position of wall~66 in response
: ~ : '
, ,
3~
-16-
to the continuously sensed level of metal in casting
chamber 70 to maintain the desired uniform depth of
metal in chamber 70.
As most clearly seen in Fiy. 3, with the casting
wheel 12 being driven in the direction indicated by
the arrow 74, a clean cool area of the outer peripheral
casting surface 1~ will continuously be presented at
the bottom portion of casting chamber 70 and move
progressively upwar~` through the molten metal in the
chamber. Since the surface 14 is cooled from the cir-
culating cooling fluid inside the casting wheel, the
molten metal which initially wets the surface as it
enters the bottom of the castlng chamber is immediately
quenched and solidified, with heat extraction continuing
and the thickness of the solidified strand adhering to
the surface 14 progressively increases throughout the
path through the molten metal in chamber 70 to the
point of emergence from the top surface of the molten
metal, thereby progressively withdrawing metal from
the chamber 70 at a substantially uniform rate. The
withdrawn metal is replaced, at an equal rate, beneath
the bottom edge of the movable wall 66 from the surge
cha~nber 68. The rate of flow beneath the metering wall
~63~
66 will, of course, depend upon the fluid head between
the surge chamber 68 and the casting chamber 70 and
the vertical position of wall 66 required to maintain
a constant depth of metal in chamber.70 will therefore
vary in response to changes in the level of metal in
the surge chamber 68. Other arrangements could, of
course, be provided for maintaining the desired depth
of metal in the casting chamber.
With a uniform flow of metal into the casting
chamber 70, the level of metal in surge chamber 68
will depend upon the level of metal in the recelving
chamber since the rate of flow beneath the fixed wall
60 will also depend upon the difference in the level of
metal in the surge chamber 68 and in the receiving
chamber 62.
Molten metal flowing into receiving chamber 62
from the open top can result in substantial turbulence
: in this chamber. Also, any slag, dross, oxides or
other material normally floating on the surface of
meta:l in the receiving chamber may be entrained in the
incoming stream of metal and.be mixed with the molten
metal in the chamber. Thus, a g`reater depth of metal
is desired in the receiving chamber to permit any
~2~32~
-18-
such entralned material to migrate back to the top
surface of the molten me-tal rather than to be carried
beneath the wall 60 into the surge chamber 68. Also,
by maintaining an excess of material in receiving
chamber 62, an empty supply ladle 64 may be removed
and replaced without interrupting the casting operation.
The apparatus and process thus far described are
substantially identical to the prior axt apparatus
and process illustrated in Fig. 7. As shown in
Fig. 7, however, this known system employs a sub-
merged metering weir 72 which acts somewhat in the
nature of a squeegee or doctor blade to control the
amount of liquid or congealing, semi-solid metal with-
drawn from the molten metal bath on the casting wheel.
Metering weir 72 has its lower contoured edge surface
submerged in the molten metal in casting chambe~r 7G
and is supported for adjustment both verticall~ and
horizontally to control both the thickness of the strand
withdrawn from the metal bath and the length, in the
direction of rotation,of the casting surface 14 exposed
,
to the molten metal supply at any instant.
While the prior-art melt drag system illustrated
in Fiy. 7 may be operated to cast metal strip, such
-
, ' :
~;~63~
--19--
systems generally have not been commercially acceptable
for high speed casting of thin, wide metal strip. For
example, in a high speed casting operation the abrasive
action of the molten metal tends to quickly wear the
S critical surface of the metering weir. Further, any
slag, dross or other material on the surface of the
molten metal in the casting chamber tends to be drawn
beneath the weir to adhere to the surface of the cast
strand, or to be embedded in the strand. Such material
also tends to produce uneven wear on the metering edge
of the weir or to adhere to the weir surface and
produce an unacceptable top surface finish on the
strip or to produce unacceptable variations in th1ckness
longitudi~ally of the strip and in transverse thickness
profile. Other prior art systems employing a metering
noæzle or the like suffer from the same drawbacks.
In accordance with the present invention, an air
knife assembly, indicated generally at 76, is employed
in place of the metering weir of the prior art apparatus
shown in Fig. 7. The construction of nozzle assembly
76 ma~ be similar to that conventionally employed in
hot dip coating of strip metal such as a galvanizing or
alumir,izing operation. Thus, as shown in Figs. l - 5,
2~
-20-
the nozzle assembly 76 comprises an elonyated manifold
structure 78 extending transversely of tundish 18,
with the manifold being made up of a pair of opposed
die members 80, 82 retained in assembled relation by
bolts, not shown, extending through openings along the
closed back and end portions of the manifold. A
thin, dimensionally stable shim member 84 is positioned
between the two die members and extends along the end
and back walls, thereby providing an elongated thin
outlet nozzle 86 communicating with the hollow interior
or plen~n chamber 88 defined by recesses in the
opposing surface of the two die members. The interior
walls of the plenum chamber preferably are smooth
and contoured to provide a su~stantially uniform
' 15 gas pressure and flow rate through outlet nozzle 86
along the full length of the outlet. This flow rate,
however, can be varied along the length of outlet 86
to provide the desired shape or top surface profile
across the width of the cast strip. This may be
,
accon~plished by contouring the surfaces of the two die
members defining the outlet opening to provide slight
variations in outlet dimensions along the nozzle length.
Alsor shims 84 of different thicknesses may be employed
-21-
to vary the flow rate from the nozzle assembly, and the
thickness of the shim may be varied along the length of
the manifold assembly, with the connecting bolts de-
flecting the dies sufficiently to conform to the shim
thickness to control the shape of the outlet nozzle and
thereby the jet of gaseous fluid discharged from the
nozzle outlet. By producing an lncreased gas flow from
the outlet 86 adjacent the side edges of the strip 16,
the strip thickness can be slightly reduced in this
area to facilitate coiling and to produce a more
desirable strip thickness profile for any subsequent
rolling required.
An inlet opening 90 formed in the die member 80
communicates with a fluid supply conduit 92, preferably
through a diffuser section 94 rigidly joined, as by
welding, to the die member 80. Alternatively, the inlet
may be at another location such as the back or end of
the manifold assembly and plural inlets may be employed
if desir d.
As shown in Fig~ 1, air knife assembly 76 is
supported above casting wheel 12 with the outlet nozzle
86 extending in parallel relation to the axis of the
casting wheel and spaced from the casting surface 14.
Suitable means such as the schematically illustrated
3Z~
22-
adjustable rack and pinion support block 96 supports
the nozzle assembly for vertical movement and a second
schematically illustrated rack and pinion assembly 98
supports the nozzle for horizontal movement in a plane
parallel to the top surface of the molten liquid in
tundish l$. Third adjusting means, such as the
schematically illustrated adjusting screw lO0 is pro-
vided for rotating the nozzle assembly about its hori-
zontal support axis in the support brackets 102.
In operation of the system, gas is supplied to the
- plenum chamber at a relatively low pressure, with the
volume of the plenum chamber being sufficient to assure
that pressure within the chamber is substantially
uniform throughout in order to provide the desired, care-
fully controlled flow through the narrow outlet nozzle
86. Preferably an inert gas such as nitrogen or argon
is employed to avoid oxidation of the hot metal by the
~jet, although air or steam may be employed for casting
: ~ some metals. In casting metals such as aluminum having
.: :
;20 :~a high affinity for oxygen, a hood may be placed over~
the 1.undish 18 and an inert gas circulated through the
:~ hood.
` As shown in Figure 2, the gas jet issuing from
: :
: nozzle 86 is directed along a plane indicated at 104
~2~3~
which intersects the surface of the emerging strip 16
at an acute angle C~ with a plane tangent to the strip
adhering to the casting wheel, the angle G~ being
measured on the side of the jet opposite the casting
pool 20. Also, the line 104 intersects the emerging
strip surface at a point slightly below the normal top
surface 106 of the molten metal in the casting chamber
70. This arrangement results in a component of force
from the gas jet extending perpendicular -to the emerging
strip 16 and a second component in a direction generally
parallel to the molten metal surface 106, with the
result that the impinging gas jet tends to be deflected
in the direction of casting chamber 70.
At the point of impingement of the gas jet onto
lS the emerging solidifying strip, the strip has an upwardly
directed (top) liquid surface, i.e., a surface which is
wet with liquid metal being carried by momentum of the
strip from the pool whereas the bottom surface c~ntact-
ing the casting surface 14 is completely solidified.
At this point, the strip beneath the liquid surface is
still very hot and soft so that the shear force from the
jet must be kept sufficiently low and controlled to avoid
damage to the surface. At the same time, the jet must
maintain proper control of the boundary between the
- . .
~:~632~
-24-
issuing s-trip and the casting pool from which the solid
strip i5 issuing. By directing the jet to a point
slightly below the casting pool surface at its forward
edge, i.e., the edge adjacent the issuing strand, the
gas pressure may be controlled to create a stable stand-
ing wave at the casting pool surface and the backward
sweeping angle of the jet causes the gas to sweep over
the molten metal surface of the wave. ~his flow of gas
effectively sweeps oxides, slag, and the like away from
10 ~ the issuing strand with the result that the cast strip
is substantially free from inclusions and surface ad-
hesions.
It has been found that relatively low pressures
are required in the air knife plenum chamber to produce
good strip surfaae and thickness profile on strip cast
using the above-described apparatus. Pressures of about
4 to about 70 gm. per sq. cm. have been successfully
used in the casting of aluminum, with somewhat higher
pressures being acceptable for some heavier metals such
as steel. The optimum gas pressure will ~ary somewhat
with other factors including the vertical position of the
outlet nozzle 86 and the angle of the jet relative to
the casting pool surface, the angle C~ , the depth of the
casting pool, and the position of the casting pool on
-25-
the casting wheel. Depending on these factors as well
as the -type of metal cast, gas pressure between about
3 and 400 gm. per sq. cm. may be used.
In the configuration of the invention described
above, the top surface of the casting pool is located
between the three and twel~re o'clock positions on the
casting wheel (as viewed in the drawings), with the tun-
dish lip, i.e., the top surface edge of bottom waIl 50
normally being located between the one o'clock and two
o'clock positions. The ideal location of the casting
pool will also depend on various factors and conditions
including the metal being cast, the diameter and speed
of the casting wheel, the depth of metal in the casting
pool and the desired thickness of the strip cast.
15The speed of the casting surface will effect both
the thickness of the strip to be cast and the tendency
of the liquid metal to be carried from the casting pool,
on the solidifying strip surface. Casting speed will,
therefore, also affect the angle CX and/or the gas
pressure requlred to produce the necessary component of
- force from the jet in a direction normal to the surface
~of the emerging strip at the point of emergence to limit
the amount of momentum liquid carried from the pool.
~ The angle a and the gas pressure will also affect the
... ~ , . . .
.
3~2~
-26-
standing wave established in the surface of the casting
pool. Thus, for any given casting wheel speed, the
horizontal and vertical position of the air knife and
-the angular direction of the jet must be coordinated
with the depth of metal in the casting pool, the
operating gas pressure in the air knife plenum chamber,
and the location of the tundish lip around the periphery
of the casting wheel surface to produce a stable stand-
ing wave in the casting pool surface and maintain the
desired thickness, profile, and top surface condition
on the emerging strip.
Casting speed and casting pool depth also influence
the point of reIease of the cast strip from the casting
surface. If casting speed is too slow for the casting
pool depth, thermal contraction of the solidifying strand
can result in release prior to emerging from the casting
pool and cause uneven or uncontrolled surface thickness
or even remelting and breaking of the strand. On the
oth~r hand, excessive~casting wheel speed can result in
~ lnsufficient solldification time and an unfavorably high
ratio of liquid-to-solid product emerging from the
.
casting pool, whereby the capability for process control
may be impaired or lost.
'
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--27--
The apparatus described above has been employed
to successfully cas-t steel and aluminum strlp over a
wide range of cas-tiny speeds. Further, by controlling
the pressure within the air knife plenum chamber and
the other parameters as discussed above, it has been
demonstrated that strip having a thickness varying over
a wide range çan be produced at speeds throughout the
range tested, and good strip quality and surface finish
have been achieved at various casting speeds. Strip
thicknesses may vary from about 0.005 mm. to more than
0 t 0 mm. and casting speed may be in the range of from
about .25 to about 30 meters per second.
Numerous trial runs have also heen made employing
castin~ wheels having casting surfaces of different
,
materials and surface characteristics. Casting sur-
faces made of copper, copper-1% chromium alloy, steel
and aluminum alloy have been employed, and bot~ smooth
and roughened or grooved casting wheel surfaces have
. been.used. Metals cast during these trlal runs have
includ~d aluminum alloys A356.2, 1100 and 3105, OFHC
copper and low carbon steel.
~;32~
-28-
Experimental runs have also been made to evaluate
the effect of casting speeds, casting pool depth, i.e.,
the length of casting wheel surface in contact with the
molten metal, air knife position and gas pressure,
S variations in angle ~ , and other variables as discussed
hereinabove. The examples contained in Table 1 below
illustrate some of various experimental runs which have
been made employing the invention.
~63~
--29--
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-30-
Experimental runs have also been made utilizing
apparatus in which a metering weir was employed instead
of the air knife, as illustrated in Fig. 7, but these
runs did not produce an acceptable cast strip. For
example, strip produced on this prior art apparatus
frequently had excessive inclusions and surface ad-
hesions, uneven thickness profile and surface damages
resulting from uneven wear on the metering weir.
Conversely, strip produced in accordance with the
present invention is of a quality adequate for some
end uses such as the production of building and rain
products and the like without further ~rocessing, and it ;
may readily be further processed by rolling since it
is substantially free of inclusions and surface adhesions.
While a preferred embodiment of the invention has
been disclosed and describecl, it should be understood
that the invention is not so limited and that it is
intended to include alI embodiments which would be
apparent to one skilled ln the art which come within
ZO the spirit and scope of the invention.
.
~ .
.
